Turbomachines, such as centrifugal compressors, in process gas service generally require rotary shaft sealing to prevent the process gas from escaping the compressor casing in an uncontrolled manner into the atmosphere. Typically, multi-stage “beam” style compressors require two seals, each disposed at an end of the rotary shaft, whereas single-stage, “overhung” style compressors require a single rotary shaft seal disposed directly behind the impeller. In the past, oil film seals were used in many applications to prevent the leakage of the process gas; however, the use of non-contacting gas seals, commonly referred to as dry gas seals, in place of oil film seals has increased dramatically in recent years.
Generally, dry gas seals function as mechanical face seals and include a mating (rotating) ring and a primary (stationary) ring. During operation, grooves in the mating ring generate a fluid-dynamic force causing the primary ring to separate from the mating ring creating a “running gap,” typically 3-10 microns, between the two rings. A sealing gas may be injected into the dry gas seal, thereby providing the working fluid for the running gap of the dry gas seal, the working fluid forming a fluid film providing a non-contacting seal between the atmosphere or flare system and the internal process gas of the compressor. The sealing gas injected into the dry gas seal may be process gas fed from the discharge line of the turbomachine. The fluid film may eliminate the need for oil and/or liquid lubricants by using the sealing gas as the lubricant. A dry gas seal running without additional lubricants may reduce and/or eliminate the contamination of the process gas by seal lubricants.
The performance of a gas seal may be described by leakage, power consumption, pressure distribution, gap, stiffness, and/or alternative measurements. Standard gas seals include spiral groove patterns optimized for rotation in a single direction. Generally, if those seals are rotated in the opposite direction during operation, the grooves will not create the desired fluid film. Standard gas seals, as described by any of the above measurements, may show significantly reduced performance and/or failure if the seals are rotated in the opposite direction.
What is needed, then, is a gas seal providing efficient performance regardless of the direction of rotation.